1. Field of the Invention
The present invention relates to a battery feed circuit, more particularly to a batter feed circuit above to be provided in a switching system line circuit.
In general, battery feed circuits are placed at the switching system side for the principal purpose of supplying DC current through subscriber lines (line A and line B) to the subscriber telephone terminal equipment to operate the same. These battery feed circuits are connected to the terminals of the line A and line B forming the above subscriber lines, so they must be designed so that the input impedance with respect to the AC signal seen from the line A and line B becomes a predetermined value. This input impedance is divided into two types (Z.sub.DT, Z.sub.CT) and must satisfy the following conditions:
[1] A high AC impedance Z.sub.DT with respect to a differential signal (voice signal),
[2] A low AC terminating impedance Z.sub.CT with respect to the in-phase signal (AC induction and other undesired AC signals),
[3] A value of the DC battery feed resistor able to supply the DC current required by the telephone, for example, several hundred ohms.
The AC terminating impedance Z.sub.DT with respect to the differential signal is preferably a high one so as to prevent attenuation of the same. On the other hand, the AC terminating impedance Z.sub.CT with respect to the above in-phase signal is preferably a low one to cause extreme attenuation of the same. Further, in recent years, the conditions for in-phase signals have become severer and a demand has arisen for prevention of distortion in the voice signals even when the in-phase signal current becomes larger than the DC battery feed current.
2. Description of the Related Art
As related art, mention may be made of Japanese Unexamined Patent Publication (Kokai) No. 57-42263. This satisfies the above conditions [2] and [2], but cannot deal with the above-mentioned recent trends, i.e., the demands for prevention of distortion in voice signals even when the in-phase signal current becomes larger than the DC battery feed current. The reasons for this is that the battery feed circuit shown in the publication is comprised of a PNP transistor with a collector connected to line B and an NPN transistor with a collector connected to line A, the former PNP transistor only transmits a DC current from its collector, and the latter NPN transistor only absorbs the DC current from its collector, i.e., conductance of the DC current in the reverse direction is not possible.
Therefore, a battery feed circuit which can not only handle DC current to line A and line B in the forward direction, but also handle DC current in the reverse direction has become necessary in practice. That is, a bidirectional battery feed circuit has become necessary. This bidirectional battery feed circuit will become needed more and more in the market in the future.
Various methods are conceivable for realizing a battery feed circuit. As one example, there has been proposed a bidirectional battery feed circuit using a transconductance amplifier. This is shown as FIG. 2 on page 262 of the IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-16, NO. 4, AUGUST 1981 (details given later). However, battery feed circuits share peripheral circuits (for example, bias circuits) and function in the line circuit, so even if a bidirectional battery feed circuit per se could be realized, it would have no practical significance.